1. Field of the Invention
The present invention relates to a rear derailleur structure which is affixed to the rear fork of a bicycle. The rear derailleur structure disclosed in the present invention utilizes a cam member to provide uniformity of operational force as the rider shifts the driving chain from one sprocket to another sprocket of a multistage sprocket assembly mounted on the bicycle frame. The improved rear derailleur structure disclosed in the present invention can, at the same time, reduce the possibility of indexing error of the derailleur caused by the protruding length of the cable adjusting screw, as often observed in the prior art derailleurs.
2. Description of the Prior Art
In recent years, the structure of most of the derailleurs taught in prior art mainly includes a fixed member, a moveable member, a pair of mutually parallel linkage, an upper guide pulley and a lower guide pulley, and a return spring. The two ends of the linkage set are connected to the fixed member and the moveable member, respectively, by a rotating pin. The upper guide pulley and the lower guide pulley are connected to the chain guide to form a guide pulley set. And the set of guide pulleys are connected to the moveable member by the rotating pin.
One end of shifting cable is fixed on the shifting lever, and the other end of the shifting cable passes through a cable adjusting screw and is then fastened on the fixed screw of the shifting cable which is settled on the outer linkage. By pulling the shifting cable for changing speed, the inner and outer linkages of parallelogram mechanism rotate around the pin of the fixed member to cause the moveable member and the set of guide pulleys to be moved outward or inward along with axis of freewheel assembly and thereby move the guide pulley to the alignment position of a respective sprocket of the freewheel assembly in order to shift a driving chain and thereby change speed.
When the rider wishes to shift the driving chain from a smaller sprocket to a larger sprocket, the pulling effort exerted by the shifting cable should be able to overcome the resistance exerted by the return spring of parallelogram mechanism (i.e., should be large enough to change the morphology of parallelogram mechanism), and thereby be able to move the guide pulley set and the driving chain inwardly along the axis of the freewheel assembly. When the rider wishes to shift the chain from the larger sprocket to the smaller sprocket, by means of loosening the shifting cable, the return force exerted by return spring will move the guide pulley set outwardly along the axis of the freewheel assembly in order to shift the chain and thereby to change speed.
The prior art is designed according to the operational principle mentioned above. However, in practice use, the defects of the control performance and indexing error of derailleur under the pre-determined stroke of index shifter can be explained as follows:
(1) As shown from FIGS. 1A through 1F, in the prior art devices, the actual pulling angle of cable (.alpha.) varies during the shifting procedure. When the chain is shifted from a smaller sprocket to a larger sprocket, the distance between the fixed screw of the cable and the adjusting screw cable becomes closer and thereby the active length of cable become shorter. When the guide pulley has a certain upward movement at the axis of freewheel assembly, the actual pulling of cable changes too much and the portion of the cable leaving the adjusting screw becomes folded so that friction and pulling effort of cable to shift chain increase rapidly. Therefore, it will cause the rider to feel the shifting operation uncomfortable. In the meantime, the shifting cable will be easily broken down by long-term friction. PA1 (2) The effect of protruding length L of adjusting screw 20 of prior art on the active length (L1, L2) of cable 21 is shown in FIG. 2A. When the adjusting screw is being screwed in and out for making the adjustment for guide pulley's initial position, the different protruding length of cable adjusting screw 20 will change the active length L of steel cable, wherein initial actual pulling angle of cable (.alpha.1, .alpha.2) be changed. It indicates that the relationship between the change of active length of cable in guide pulley's working area and the change of position of guide pulley changes. Therefore, the chain guide and the guide pulley will often fail to move to the appropriate position as intended within a certain change of cable by using the index type of shifting lever. The function for changing speed will thereby become lower and will even fail.